In present high definition television (HDTV), and satellite broadcast systems, programs comprise combinations of a video signal representing a moving image, one or more audio signals (for stereo or multilanguage capability), and one or more data signals (for closed captioning, and/or interactive computer program code and/or data). As a specific example, a system proposed by the Grand Alliance consortium provides for a program to contain a video signal component, two audio signal components and four auxiliary data signal components. A stream of successive packets of data is formed, each packet containing data from one of the component signals. In this manner the seven component signals are time multiplexed into a single packet stream, which is broadcast over a tranport link.
Remote locations receive and process the data contained in the packet stream to reproduce the seven component signals. The image represented by the video signal component is displayed on a display screen, and the sound represented by the audio signal component(s) is reproduced on speakers. The auxiliary data component signals are processed by appropriate circuitry at the remote location, and used as intended. For example, if one of the auxiliary data component signals represents closed captioning information, a video signal representing the closed caption image is generated, that image signal is combined with the image signal representing the video signal component, and the image represented by the combined image signals is displayed on the display screen.
The device used to combine the seven component signals is called a transport stream encoder. A working draft published by the International Organization for Standardization, ISO/IEC JTC1/SC29/WG11, Coding of Moving Pictures and Associated Audio, entitled "MPEG-2 Systems Working Draft", (ISO/IEC/JTC 1/SC29/WG 11/N0531) in September 1993, illustrates a block diagram of a transport stream encoder. This block diagram includes an input FIFO and data selector for temporarily buffering data representing the seven digital data signal components and producing data representing a selected one of the seven signal components. This data is supplied to a packetizer. A scheduler selects which of the signal components is to be carried in the next packet slot using either a priority scheme or a fixed time slot scheme, both of which are described in more detail below. To form a packet containing data from a selected signal component, the scheduler conditions the data selector to couple the output terminal of the input FIFO for the selected signal component to the packetizer, and conditions that input FIFO to produce the data to be carried in that packet. The resulting packet is transmitted over the transport link.
A problem with the transport stream encoder illustrated in the above working draft is that the number of signals which may be combined in the transported packet stream is limited to the seven referred to above. In future developments, it will probably be required to transport more than these seven signal components over the transmitted packet stream. However, the system in the working draft is difficult to expand. For each additional signal component a FIFO must be added, the data selector must be redesigned to include an added input terminal, and the circuitry for selecting that added component and coupling the output of that added component's FIFO to the packetizer must be similarly expanded.